1. Field of the Invention
The present invention generally concerns the control of a reflected light beam by the shape of a reflecting surface and is applicable to various optical fields with particular relevance to lighting equipment. The invention is important to vehicle headlights and, in particular, reflectors therefor which are capable of producing a low intensity beam having a sharp cutline while using its entire reflecting surface. The invention is especially applicable to headlights for streamlined automobiles.
2. Description of the Related Art
FIG. 25 is a diagram showing the basic construction of a low beam headlight for an automobile. A coil-like filament c is disposed adjacent to the focal point b of a paraboloid-of-revolution reflector a such that the central axis of the filament c extends along the optical axis of the reflector a (so-called C-8 type filament arrangement). Below the filament c is a shade d that serves to form a cutline (or cutoff) in a light-distribution pattern. A sharp cutline is desirable for an automobile headlamp because it permits accurate adjustment of the lamp so that there is illumination of the road ahead of the vehicle by light from below the cutline but there is no illumination above the cutline that may "dazzle" oncoming vehicles.
As is understood from the figure, since part of light emitted from the filament c is shielded by the shade d, no light reaches a surface a.sub.L (indicated by hatching) which occupies almost the entire lower half of the reflecting surface of the reflector a. That is, such part of the light is cut by the shade d, and is not utilized. As a result, the utilization rate of the luminous flux from the lamp is reduced.
Hence, a pattern f projected on a screen e that is disposed in front of the reflector a at a predetermined distance away therefrom is formed into an almost semicircular pattern, in which one part g of its cutline forms a predetermined angle (15.degree.) relative to a horizontal line (this line is indicated by "H--H", the vertical line is indicated by "V--V", and their intersection is indicated by "HV"), and the other part h of the cutline extends in parallel with and below the horizontal line H--H.
If the emitted light pattern is further subjected to light-distribution control by diffusion lens steps of an outer lens (not shown) disposed ahead of the reflector a, the low beam distribution pattern is formed into a pattern i, as shown in FIG. 26, which is elongated in the horizontal direction.
The headlamp design of FIGS. 25 and 26 are not suitable for modern styling requirements. In recent years, the bodies of automobiles have become "streamlined" in order to satisfy the demand for sleek styling as well as efficient aerodynamic characteristics and design. As a result, it is required that headlights be designed to match the so-called "slant-nosed" front part of the body. In response to such a requirement, often headlights are designed so that they are horizontally narrower (i.e., the vertical height of a headlight is decreased), and that they have a larger slant (i.e., a so-called slant angle, formed between the outer lens and the vertical axis, is increased).
If the vertical height of the reflector is decreased and if the outer lens is largely inclined, then the outer lens should no longer be provided with wide diffusion lens steps. If such steps are still used, the so-called "light tailing" phenomenon may be observed in which the right and left end portions of a light-distribution pattern have a gentle slope. These requirements impose major design restrictions.
To overcome this problem, it has been suggested that the light-distribution control function conventionally assumed by the outer lens should be undertaken by the reflector. To cope with the narrowing of the lamp height, it is desirable to remove a shade to prevent a reduction in luminous flux utilization rate, and to fully use the entire surface of the reflector.
A variety of reflectors having such a light-distribution control function have been proposed. One example is a reflector j whose reflecting surface k is divided into two paraboloid-of-revolution reflecting regions k.sub.H, k.sub.L that substantially occupy the upper and lower halves, respectively, as shown in FIG. 27(a). And as shown in FIG. 27(b), the rear end of a filament c is positioned at a point displaced ahead by .alpha. (i.e., in the direction of leaving from the reflector) from the focal point F of the upper reflecting region k.sub.H, while the front end of the filament c is positioned at a point displaced behind by .beta. from the focal point F.sub.2 of the lower reflecting region k.sub.L. Both focal points are on the optical axis +X-X of the reflector j.
In this case, a composite pattern m to be projected by the reflector j on a distant screen, as shown in FIG. 28, is formed into a shape in which a pattern n (indicated by the solid line) formed by the upper reflecting region k.sub.H and a pattern o (indicated by the one dot chain line) formed by the lower reflecting region k.sub.L are combined. As is understood from FIG. 28, the "cutline" of the pattern m is formed by the upper edge of the pattern n.
In the aforesaid reflector j, its entire surface is utilized. However, the quantity of light in regions A, A adjacent to the cutline is relatively small compared with that in region B where the patterns n and o overlap. Accordingly, the distribution of light is not uniform and the brightness of the projected light gradually changes (is reduced) as the position nears the cutline. As a result, it is difficult to form a sharp cutline.
To overcome this shortcoming, two small shades p, p may be disposed around the light source as shown in FIG. 29 so that a sharp cutline can be obtained. However, the design of such a mounting structure, etc., as to ensure positional accuracy of the shades p, p, is difficult. Further, since light beams toward the boundaries between the reflecting regions k.sub.H and k.sub.L (indicated by hatching) are shielded by the shades p, p, the effective use of the reflecting surface is not fully achieved, thus making this technique not the best solution but rather a compromise.